[PATCH v2] mm/oom_kill: show oom eligibility when displaying the current memory state of all tasks

[Aaron Tomlin]

Changes since v2:
 - Use single character (e.g. 'R' for MMF_OOM_SKIP) as suggested
   by Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
 - Add new header to oom_dump_tasks documentation


At the present time, when showing potential OOM victims, we do not
exclude tasks which already have MMF_OOM_SKIP set; it is possible that
the last OOM killable victim was already OOM killed, yet the OOM
reaper failed to reclaim memory and set MMF_OOM_SKIP.
This can be confusing/or perhaps even misleading, to the reader of the
OOM report. Now, we already unconditionally display a task's
oom_score_adj_min value that can be set to OOM_SCORE_ADJ_MIN which is
indicative of an "unkillable" task i.e. is not eligible.

This patch provides a clear indication with regard to the OOM
eligibility of each displayed task.

Signed-off-by: Aaron Tomlin <atomlin@xxxxxxxxxx>
---
 Documentation/admin-guide/sysctl/vm.rst |  5 ++--
 mm/oom_kill.c                           | 31 +++++++++++++++++++++----
 2 files changed, 30 insertions(+), 6 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index 586cd4b86428..123be642bc7e 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -658,8 +658,9 @@ oom_dump_tasks
 Enables a system-wide task dump (excluding kernel threads) to be produced
 when the kernel performs an OOM-killing and includes such information as
 pid, uid, tgid, vm size, rss, pgtables_bytes, swapents, oom_score_adj
-score, and name.  This is helpful to determine why the OOM killer was
-invoked, to identify the rogue task that caused it, and to determine why
+score, oom eligibility status and name.  This is helpful to determine why
+the OOM killer was invoked, to identify the rogue task that caused it, and
+to determine why
 the OOM killer chose the task it did to kill.
 
 If this is set to zero, this information is suppressed.  On very
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index eefd3f5fde46..094b7b61d66f 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -160,6 +160,27 @@ static inline bool is_sysrq_oom(struct oom_control *oc)
 	return oc->order == -1;
 }
 
+/**
+ * is_task_eligible_oom - determine if and why a task cannot be OOM killed
+ * @tsk: task to check
+ *
+ * Needs to be called with task_lock().
+ */
+static const char * is_task_oom_eligible(struct task_struct *p)
+{
+	long adj;
+
+	adj = (long)p->signal->oom_score_adj;
+	if (adj == OOM_SCORE_ADJ_MIN)
+		return "S";
+	else if (test_bit(MMF_OOM_SKIP, &p->mm->flags)
+		return "R";
+	else if (in_vfork(p))
+		return "V";
+	else
+		return "";
+}
+
 /* return true if the task is not adequate as candidate victim task. */
 static bool oom_unkillable_task(struct task_struct *p)
 {
@@ -401,12 +422,13 @@ static int dump_task(struct task_struct *p, void *arg)
 		return 0;
 	}
 
-	pr_info("[%7d] %5d %5d %8lu %8lu %8ld %8lu         %5hd %s\n",
+	pr_info("[%7d] %5d %5d %8lu %8lu %8ld %8lu         %5hd %13s %s\n",
 		task->pid, from_kuid(&init_user_ns, task_uid(task)),
 		task->tgid, task->mm->total_vm, get_mm_rss(task->mm),
 		mm_pgtables_bytes(task->mm),
 		get_mm_counter(task->mm, MM_SWAPENTS),
-		task->signal->oom_score_adj, task->comm);
+		task->signal->oom_score_adj, is_task_oom_eligible(task),
+		task->comm);
 	task_unlock(task);
 
 	return 0;
@@ -420,12 +442,13 @@ static int dump_task(struct task_struct *p, void *arg)
  * memcg, not in the same cpuset, or bound to a disjoint set of mempolicy nodes
  * are not shown.
  * State information includes task's pid, uid, tgid, vm size, rss,
- * pgtables_bytes, swapents, oom_score_adj value, and name.
+ * pgtables_bytes, swapents, oom_score_adj value, oom eligibility status
+ * and name.
  */
 static void dump_tasks(struct oom_control *oc)
 {
 	pr_info("Tasks state (memory values in pages):\n");
-	pr_info("[  pid  ]   uid  tgid total_vm      rss pgtables_bytes swapents oom_score_adj name\n");
+	pr_info("[  pid  ]   uid  tgid total_vm      rss pgtables_bytes swapents oom_score_adj oom eligible? name\n");
 
 	if (is_memcg_oom(oc))
 		mem_cgroup_scan_tasks(oc->memcg, dump_task, oc);
-- 
2.26.3